Measurements of systolic (SBP) and diastolic (DBP) blood pressure were made at rest and during symptom-limited exercise with an automated blood pressure measuring device (EBPM). Comparisons were made between the EBPM readings and those made with mercury manometer. Correlations were high (SBP r = 0.92, DBP r = 0.80) when readings were made in the same arm, but were less satisfactory when the cuffs were on different arms (SBP r = 0.80, DBP r = 0.46). The correlation between two mercury manometer readings was SBP r = 0.90, and DBP r = 0.75. Comparison between EBPM and intra-arterial measurements were similar (SBP r = 0.74, DBP r = 0.79) to comparison between mercury manometer and intra-arterial measurements (SBP r = 0.81, DBP r = 0.61). The EBPM detected SBP at consistently higher levels than did physicians, which may be an advantage in the noisy environment of an exercise test. There was a definite tendency for physicians to record blood pressure to the nearest 10 mm Hg, whereas the frequency distribution curve for EBPM measurements was smoother. The EBPM operated satisfactorily at rest and during maximal exercise and gave as reliable measurements as a physician using a mercury manometer and, in the small number of available cases, detected exertional hypotension more often than the physician.